The present invention relates to a method and apparatus for digitally processing stored, compressed analog signals. The present invention has a particular utility in processing analog signals, such as audio signals stored in compressed form on a storage medium such as a magnetic tape, such that the recorded signals may be transmitted either in digital or analog form, processed further and/or re-recorded.
In the past, audio recordings have often been preserved in a compressed analog manner in order to suppress noise. The analog signals are dynamically compressed, as is conventional. Such compression, of course, is well-known for improving the signal-to-noise ratio by volume (gain) compression of the analog signals so that weak signal components are not lost in the background and so that strong signals do not overload the system. To realize a natural sounding reproduction, the storage media utilized for the audio preservation were then scanned with the aid of an analog expander.
Since digital technology has become more and more accepted, there often exists the need to convert signals picked up from storage media containing analog signals, that were recorded in a compressed manner, into digital signals. The ultimate aim is to produce a natural sounding reproduction with the least amount of noise, i.e., with a large volume range.
E. Schrel,uml/o/ der, J. Wermuth, "Ein neues Kompandersystem-Grundlagen und Einsatzmoglichkeiten" [A Novel Compander System--Principles and Possible Uses], Fernseh-und KinoTechnik [Television and Cinematic Technology], Volume 30 (1976), No. 12, pages 427-429 (Schroder et al); J. Wermuth, "Kompandersystem "telcom C4"[Compander System "telcom C4"]Fernseh-und Kino-Technik [Television and Cinematic Technology], Volume 34, No. 3/80, pages 91-94 (Wermuth), and J. Wermuth, W. Schneider, "Design and Application of a New Compact Compander Card," Audio Engineering Society (AES), Preprint of the 79th Convention, 1985, October 12-16, New York (Wermuth et al) disclose two different principles of a fully analog compander system which is composed in each case of an analog compressor and a matching analog expander. Various variations thereof are disclosed.
The basic principle of an analog expander is described in Schroder et al, paragraph 3 (3.2) and paragraph 4. A more recent version can be found in Wermuth et al (FIG. 5). Also, a distinction is made between a professional version for studio applications and a simplified version for consumer use. In the professional version, the useful spectral range (0 to 20 kHz) is subdivided with the aid of filters into L partial bands (Wermuth, 4 partial bands) with each partial band having its own analog expander.
In the static case, the gain characteristic V=P2-Pl (in dB) exhibits a slope m/n which is a function of the input level P1 of an analog compressor (with P2 =output level), where m and n are whole natural integers. In the aforementioned references, m=2 and n=3. The gain characteristic of the analog expander must therefore have a slope of n/m so that the two functions supplement one another in the static case to result in a total slope (m/n).(n/m)=1.